1. Field of the Invention
The present invention relates to a method of detecting a paging channel in a multifrequency radio pager network wherein an overall service coverage area is divided into a plurality of small paging areas. More specifically, the present invention relates to such a method wherein the paging channel to be detected is dynamically assigned to one of a plurality of frequency subset indicators in a given paging area. The frequency subset indicators are related to one frequency subset number.
2. Description of the Prior Art
It is known in the art that a radio pager is a device which alerts a subscriber to call a predetermined phone number by means of sound and/or flashing light.
Recently, in order to transmit more pieces of information to a subscriber at one calling, a radio pager which is equipped with a display function, has proven very popular and is gradually replacing the above mentioned older style pagers.
A radio paging system currently in use is designed such that a pager is able to receive its paging messages, via a predetermined paging channel, throughout an entire service coverage area to which the subscriber has subscribed. In other words, there exists no radio pager system wherein a service coverage area is divided into a plurality of small paging areas and wherein a pager uses different frequencies in different paging areas.
However, merely by way of example, the European Telecommunications Standards Institute has proposed to provide frequency divided network operations in the European Radio Message System (ERMES). The frequency divided network operation implies that adjacent paging areas use different frequencies. Within one network a pager should use adjacent frequencies in adjacent paging areas.
In the frequency divided (viz., multifrequency) network, each of the pagers is informed on which channel to expect its messages. This is accomplished with a combination of a frequency subset indicator (FSI) transmitted to all pagers in a system information partition and a frequency subset number (FSN) permanently stored in the pager. The frequency subset indicator (FMI) shall be transmitted for all pagers.
FIG. 1 is a chart which depicts a format of the paging signal which is transmitted to the pagers in the ERMES. The code format Shown in FIG. 1 starts with a synchronization part which consists of a preamble word (PR) and a synchronization word (SYN). A system information partition comes next which consists of two system information (SI, SI') parts and a supplementary system information (SSI) part. The system information partition is followed by address and message partitions as shown.
FIG. 2 is a diagram which illustrates a relationship between frequency subset indicators (FSIs: 30-00) and frequency subset numbers (FMNs: 00-15), both of which have been proposed by the above mentioned ERMES. Each of all pagers shall be assigned a single FSN between 00 and 15. Each FSN defines a unique subset of five FSIs as indicated in FIG. 2. It is intended that the FSIs are used in descending order. Thus, in the event that a single channel is used within a paging area, the transmitted FSI is set to 30. On the other hand, if two channels are used within a paging area, the transmitted FSIs are set to 28 and 29. The FSI(s) transmitted is located in the system information SI' in the code format of FIG. 1.
It is assumed that the number of frequency channels assigned to an overall service coverage area is 16 and depicted by F0-F15. It should be noted that the FSIs do not correspond directly to frequency channel numbers. In more specific terms, a system operator allocates one or more of the 16 channels F0-F15 to one or more of the thirty FSIs (viz., FSI values of 31 to 00) in each paging area. Thus, the FSI/FSN arrangement renders it possible to have a dynamic number of channels in use.
The FSI broadcast on the paging channel indicates that messages wall be transmitted for pagers with an FSN in the FSI's subset. For example, when the FSI on a channel equals 27, only messages to pagers with FSNS 12, 13, 14 and 15 will be carried. Conversely, a pager with FSN=12 should look for its messages, only on one of the channels broadcasting FSI values of 12, 22, 27, 29 or 30.
Merely for the convenience of description it is assumed that:
(a) the pager with FSN=12 enters a given paging area; PA1 (b) the pager has previously stored FSI values of 12, 22, 27, 29 and 30 in a memory it is equipped with; and PA1 (c) four frequency channels F15, F10, F8 and F2 are used in the paging area in question and assigned respectively to the FSIs of 27, 26, 25 and 24 as indicated in FIG. 2.
With these assumptions, the pager with FSN=12 has to detect channel F15 which has been assigned to FSI=27, one of its five FSIs, and then fix itself one channel F15. In this end, the pager begins to search for the channel F15 by sequentially changing the tuning frequencies in a predetermined order using a frequency synthesizer it is equipped with. It is further assumed that the channel searching is implemented in an ascending order (viz., from F0 to F15) by way of example.
The pager is firstly tuned to the frequency F0. Since F0 is not in use in this particular case, the pager tuning proceeds to the next frequency F1 which is also not in use in this case. Following this, the pager is tuned to F2. However, the FSI value of 24 transmitted on the channel F2 has not been assigned to the pager in question and accordingly, the pager is subsequently tuned to the following channel frequencies (viz., from F02 to F15).
When the pager is eventually tuned to the channel F15, it detects the FSI=27 in the system information partition (FIG. 1) on the channel F15 and is fixed to same and awaits the forthcoming message reception.
The above mentioned prior art techniques have suffered from the difficulty in that the number of channel tuning operations is apt to be undesirably large before the assigned channel is detected, and in the worst case is carried out 16 times.
This problem is especially disagreeable in the case where a pager subscriber frequently moves from one paging area to another.
FIG. 3 schematically illustrates such a case wherein a service coverage area is divided into 6 paging areas A1-A6 by way of example. It is assumed that the paging areas A1-A6 have been assigned the frequency channels F0, F2-F4, F7-F8, F10-12 and F14-F15 as indicated. If a given pager moves from a point M to N along a broken line, the pager is required to search for the channel assigned thereto every time it enters the adjacent areas. As mentioned above, the pager is unconditionally, successively tuned to the channels in a predetermined order and, accordingly the tuning operations are time consuming.